The disclosed invention is in the field of liquid chromatography. Liquid chromatography is used by the art to analyze polymers, with regard to molecular size, by Size Exclusion Chromatography (SEC), and, with regard to chemical composition, by High Performance Liquid Chromatography (HPLC). This disclosure relates to HPLC analysis of polymers with regard to chemical composition.
Polyolefin polymers (such as polymers and copolymers comprising polymerized ethylene monomer and/or propylene monomer) have long been analyzed with regard to chemical composition distribution by temperature rising elution fractionation (TREF) and crystallization analysis fractionation (CRYSTAF). However, neither TREF nor CRYSTAF can be used to analyze amorphous polyolefin polymers. Furthermore, both TREF and CRYSTAF require a relatively long analysis time. Therefore, the art turned to HPLC in an attempt to reduce analysis time and to expand the scope of analysis to amorphous polymers. Macko et al. apparently were the first to do so in 2003 by studying the retention of polyethylene standards on silica and zeolite stationary phases (J. Chrom. A, 1002 (2003) 55). Wang, et al. studied the retention of polyethylene and polypropylene by zeolites in 2005 (Macromolecules, V. 38, No. 25 (2005) 10341). Heinz and Pasch used a silica stationary phase to analyze polyethylene-polypropylene blends by HPLC (Polymer 46 (2005) 12040). Albrecht, et al., used a silica stationary phase to analyze ethylene-vinyl acetate copolymers by HPLC (Macromolecules 2007, 40, 5545). Albrecht, et al., used a silica stationary phase to analyze ethylene-propylene copolymers by HPLC (Macromol. Symp. 2007, 257, 46).
Some chromatography separations using graphite are disclosed in the following references: Macko et al., Separation of Propene/1-Alkene and Ethylene/1-Alkene Copolymers by High-Temperature Adsorption Liquid Chromatography, Polymer 50 (2009), 5443-5448; Macko et al., Separation of Linear Polyethylene from Isotactic, Atactic, and Syndiotactic Polypropylene by High-Temperature Adsorption Liquid Chromatography, Macromolecules (2009), 42, 6063-6067; Chitta et al., Elution Behavior of Polyethylene and Polypropylene Standards on Carbon Sorbents, Journal of Chromatography A, 1217 (2010) 7717-7722; Findenegg et al., Adsorption from Solution of Large Alkane and Related Molecules onto Graphitized Carbon, Carbon, Vol 25, No. 1, (1987), 119-128; and Yin et al., Theoretical Study of the Effects of Intermolecular Interactions in Self-Assembled Long-Chain Alkanes Adsorbed on Graphite Surfaces, Surface and Interface Analysis (2001), 32, 248-252. See also U.S. Publication No. 2010/0093964. Other two dimensional chromatography with or without graphite are disclosed in the following: Roy et al., Development of Comprehensive Two-Dimensional High Temperature Liquid Chromatography×Gel Permeation Chromatography for Characterization of Polyolefins, Macromolecules (2010), 43, 3710-3720; and Ginzburg et al., High-Temperature Two-dimensional Liquid Chromatography of Ethylene-Vinylacetate Copolymers, Journal of Chromatography A, 1217 (2010), 6867-6874.
A remaining problem for the HPLC analysis of polyolefin polymers is the limited separation efficiency obtained by the prior art methods. There remains a need for new chromatographic methods for polyolefin polymers that provide improved separation efficiencies and reduced analysis times. These needs and others have been met by the following invention.